1. Field of the Invention
The present invention relates to a method and apparatus for determining the internal pressure of a sealed container. The method and apparatus take into consideration variables that effect such determination, and in particular, the level of contents in the sealed container.
2. Background of the Prior Art
At many waste sites, transuranic (TRU), low-level, and mixed wastes are stored in 55-gallon drums. Many of these drums contain hazardous, organic waste as well. Radiolysis or other physical or chemical processes may result in gaseous emissions inside the drums. When this happens, the pressure within the drums increases, sometimes to unacceptable levels. In the most drastic cases, the emissions inflame or explode (e.g., due to hydrogen produced from radiolysis). Currently, regulatory procedures require that each drum be individually opened and inspected for the presence of hazardous organic waste. This procedure is dangerous for workers when drastic conditions such as described above exist. A non-intrusive technique that detects an increase in pressure over ambient would alert workers of potential danger while maintaining worker safety. Such a technique would also allow rapid segregation of suspect drums, thus providing more rapid treatment of safe drums.
There are a number of documented incidences in which drums have burst, or ruptured, and spilled their contents. In some cases workers have been injured. There thus exists a need to address the safety concerns associated with the handling of sealed drums of hazardous waste.
U.S. Pat. No. 4,009,616 discloses an acoustic method for measuring gas pressure in a hermetically sealed container. An acoustic signal is transmitted into and through the walls of the container along a path through a gas medium. The transmitted signal is received after it has traveled a given path through the gas medium and converted to a corresponding electrical output which is calibrated as a direct measure of the pressure of the gas.
U.S. Pat. No. 4,187,718 discloses a method and apparatus for determining the internal pressure of a sealed container by converting a sound wave excited at the wall of the container to a detectable electrical signal.
U.S. Pat. No. 5,585,567 discloses a method and apparatus for determining the internal pressure of a sealed container, which involve striking the wall of the container in a controlled manner so as to excite at least two modes of vibration having separate frequencies f1, and f2; and detecting a vibration resulting from the striking of the wall of the container.
Thinnes, G. L., et al. (1995) discloses resonance analysis techniques to detect subtle changes in a storage container lid""s vibration characteristics caused by changes in internal pressure.
Patel et al. (1999) discloses a method for determining the internal pressure of a drum by determining a natural frequency of the drum lid. The method involves tapping the drum lid, recording the audible sound with a microphone, and converting the time-domain signal to a frequency spectrum using a Fast Fourier Transform.
The present inventors have determined that frequency measurements of a drum""s lid can be used to accurately determine its internal pressure and that the frequency measurements are a function of a number of variables, including not only the internal pressure of the drum, but also the level of contents within the drum. None of the methods and apparatus described above take these variables into consideration when determining the safety of a sealed container.
The present invention is a simple, non-intrusive method and apparatus to more accurately determine the internal pressure of a sealed container (or storage drum).
The method for determining the internal pressure of a sealed container of the invention involves:
exciting a lid of the container so as to create at least two modes of vibration having separate frequencies, wherein said frequencies are fundamental, f1, and a second frequency, f2, preferably the second axisymmetric mode;
detecting the vibration resulting from said exciting to determine f1 and f2;
using f2, which is indicative of internal pressure, to calculate a first value for internal pressure using a first mathematical model that is calibrated to the lid on the sealed container;
using f1, which is indicative of volume of contents, to calculate the volume of contents in the sealed container using a second mathematical model that is calibrated to the lid on the sealed container, wherein a natural frequency of said lid is a function of said internal pressure and said volume of contents; and
compensating for said volume of contents to determine a second value for internal pressure, wherein said second value for internal pressure is more reliable than said first value for internal pressure.
The apparatus for determining the internal pressure of a sealed container of the invention includes:
means for exciting a lid of the container so as to create at least two modes of vibration having separate frequencies, wherein said frequencies are fundamental, f1, and a second frequency, f2, preferably the second axisymmetric mode;
detecting means for detecting vibration resulting from the exciting of said container to determine f1 and f2;
calculating means for calculating a first value for internal pressure of said container using f2;
calculating means for calculating a volume of contents of said container using f1;
wherein a natural frequency of said lid is a function of said internal pressure and said volume of contents; and
calculating means for compensating for said volume of contents to determine a second value for internal pressure, wherein said second value for internal pressure is more reliable than said first value for internal pressure.
The apparatus of the invention may be a laboratory instrument or a simple handheld instrument that works in real-time.
The present invention is illustrated below in greater detail with reference to non-limiting examples and accompanying drawings. It should be understood, however, that the present invention is not to be construed as being limited thereto.